Through their ability to alter gene expression and affect cell proliferation and differentiation, 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD) and related xenobiotics are potent immunogenic, reproductive, and developmental toxicants. They produce these effects by binding to a gene regulatory protein, the Ah receptor (AhR), to inappropriately modulate gene expression. It is not known which developing tissues are most sensitive to these compounds and how this sensitivity varies during critical developmental periods and/or during critical times of exposure. The normal function of the AhR has not been delineated and an endogenous ligand has not been identified. Through a determination of the tissue- specific transcriptional activity of the AhR complex in vivo it will be possible to determine: 1) in which developing issues the AhR complex has normal function, and 2) in which tissues the exposure to TCDD may disrupt this process. Thus, it will be possible to determine the target tissues for TCDD toxicity and the critical periods of development that are most sensitive. The purpose of this project is to determine the tissue and cellular sites and time of AhR stimulated transcription activity in developing animals utilizing a transgenic mouse model. A developed hemizygous mouse model will be bred to homozygosity. The presence and inheritability of the transgene will be determined by RT-PCR and Southern blotting. Utilizing various mouse models containing a responsive transgene, studies will determine whether the developmental expression of the transgene is or is not dependent on the site of gene integration. Studies will examine the ability of the transgene to be induced following exposure of young adult animals to TCDD. These data will be compared to the level of transcriptionally active AhR complete and the inducibility of other genes. Dose-response studies will determine sensitivity. These studies will verify that the expression of the transgene is related to the presence of the active AhR. By sensitive in situ staining techniques, the developmental expression of the transgene in tissues and cells will be determined in the absence of TCDD exposure. Results will be compared to immunohistochemical studies to determine the presence of AhR protein and its heterodimeric partner, Arnt, in these tissues. Additional studies will determine the modulated expression of the transgene following exposure of developing animals to TCDD.